1. Field of the Invention
The present invention relates to a discharge tube having a pair of discharge electrodes.
2. Description of the Related Art
In order to generate a discharge phenomenon in a space, it is necessary to apply a voltage exceeding a discharge starting voltage to the space. For this purpose, a circuit for generating a high-voltage trigger may be required. For example, an HID (High Intensity Discharge) recently used for a vehicle headlight and the like requires an ignition circuit for generating a high-voltage trigger used to start discharge. The ignition circuit is mainly configured with a capacitor for storing charge, a transformer for generating the high-voltage trigger, and a discharge tube (sometimes referred to as a switching discharge tube) for generating a stable power supply pulse (for example, refer to JP-A-2004-220808 and JP-A-2004-235017).
FIGS. 6 and 7 show a discharge tube 100 as an exemplary discharge tube used in the related art. FIG. 6 is a cross-sectional view of the discharge tube 100, and FIG. 7 is a perspective view showing an external appearance of the discharge tube 100.
Referring to FIGS. 6 and 7, the discharge tube 100 is roughly configured with an airtight cylinder 110, an upper discharge electrode 122, a lower discharge electrode 124, filler gas, and the like. The airtight cylinder 110 in which a discharge space 100a is formed has a cylindrical shape, and is formed of an insulating material such as a ceramic material.
An upper discharge electrode 122 and a lower discharge electrode 124 are attached at a top end opening and a bottom end opening of the airtight cylinder 110 respectively. The upper discharge electrode 122 and the lower discharge electrode 124 are integrally formed with a disc-shaped cap portion 126 and a disc-shaped cap portion 128, respectively.
A metalized face 140 is formed at the top end opening and the bottom end opening of the airtight cylinder 110. The cap portions 126 and 128 formed at the upper discharge electrode 122 and the lower discharge electrode 124 are brazed to the metalized face 140 formed at each opening of the airtight cylinder 110. Thus, the upper discharge electrode 122 and the lower discharge electrode 124 are attached to the airtight cylinder 110. When they are attached together, filler gas is sealed in the airtight cylinder 110. In this way, the discharge space 100a where the filler gas is sealed is formed in the airtight cylinder 110 by the airtight cylinder 110, the upper discharge electrode 122 and the lower discharge electrode 124.
The upper discharge electrode 122 protrudes from the cap portion 126 toward a center position of the airtight cylinder 110, and its tip portion is formed in a cylindrical shape having small diameter. An upper discharge electrode face 123 is formed at the tip portion of the cylindrical shape having small diameter, and a concave portion 127 is formed on the upper discharge electrode face 123 for generating a stable discharge.
Similarly, the lower discharge electrode 124 protrudes from the cap portion 128 toward the center position of the airtight cylinder 110, and its tip portion is formed in a cylindrical shape having small diameter. A lower discharge electrode face 125 is formed at the tip portion of the cylindrical shape having small diameter, and a concave portion 127 is also formed on the lower discharge electrode face 125 for generating a stable discharge. On the upper discharge electrode face 123 and the lower discharge electrode face 125 is applied copper plating, for example.
Discharge of the discharge tube 100 is generated at a discharge gap 129 as a separated portion between the upper discharge electrode face 123 and the lower discharge electrode face 125 in the discharge space 100a. 
Lead wires 112, 114 are respectively connected to the upper discharge electrode 122 and the lower discharge electrode 124 for easy connection of the discharge tube 100 with another device.
Furthermore, a variety of materials may be used for a discharge tube. In addition to the structure described above, a discharge tube made of glass is proposed, for example. In this case, coating for breakage prevention is applied to a surface of the glass so as to prevent possible breakage of glass when the glass is mounted on a board by an insert machine (for example, refer to JP-A-9-63742).
In many cases, discharge tube has been used in a device placed indoors where environmental condition around the discharge tube does not change largely. However, recently, environments in which the discharge tube is used are not limited to certain conditions but may be diverse such as the discharge tube used for a vehicle headlight. A predetermined electrical characteristic may not be obtained under harsh conditions.
For example, in a case where the discharge tube is used under a condition of high temperature and high humidity, moisture may adhere to the airtight cylinder shown in FIGS. 6 and 7, thus deteriorating an insulation resistance and an electrical characteristic (switching characteristic) of the airtight cylinder. Moreover, in case where a sudden change occurs in an external temperature, the airtight cylinder may produce dew condensation and the adhered moisture may deteriorate the insulation resistance.